1. Field of the Invention
The present invention relates to an infrared detector which is capable of absorption and detecting infrared radiation from various objects, and more particularly, it relates to a bolometer type infrared detector for detecting infrared radiation radiated from object bodies by using materials incorporated in the infrared detector whose resistance value is changed based on the change of temperature value caused by absorption of the infrared radiation.
2. Description of the Prior Art
A bolometer type infrared (IR) detector absorbs infrared radiation from various objects. A resistance value of the section made up of a resistance material in the bolometer type IR detector is heated by the absorption of the IR radiation. This temperature change causes the change of the resistance value of the resistance material. The bolometer type IR detector detects and absorbs the existence of the object that radiates this IR radiation by measuring the change of bias current or bias voltage applied to the resistance material.
FIG. 13A is a plan view of a detection element section in a conventional bolometer type IR detector which is disclosed in literatures, for example, disclosed in the literature of U.S. Pat. No. 5,260,225. FIG. 13B is a sectional view showing a configuration of the detection element section, as shown in FIG. 13A. In the same diagrams, a reference number 33 designates a resistance material section. A reference number 12 indicates a lead that is made up of a metal through which a bias voltage or a bias current is applied to the resistance material section 33. A reference number 3 denotes an insulating thin film that is made up of an insulator material such as SiO.sub.2, for example.
In this detector element in the conventional bolometer IR detector shown in FIG. 13B, a hollow section 34 is formed under the lower section of the insulating thin film 3 in order to increase effectively the temperature of the resistance material section 33 caused by the incidence of IR radiation to the resistance material. In general, a metal thin film such as a platinum (P) and a titanium (Ti), or a ceramic such as a vanadium oxide, or a semiconductor such as a poly-crystal silicon and an amorphous silicon is used as the resistance material 33.
The Temperature Coefficient of Resistance (hereinafter referred to as "TCR") is designated as follows: EQU (1/R).times.(dR/dT)[%/k],
where R is a resistance value of the resistance material section and T is the absolute temperature. In general, The TCR value of a metal thin film has +0.1 [%/k], a semiconductor has -2.0[%/k], and a vanadium oxide (VOx) has -2.0 [%/k] or more.
Using vanadium oxide having a large TCR value has the benefit of increasing sensitivity of the bolometer type IR detector. However, there is also a possibility of causing a contamination of devices and the like. The vanadium oxide is therefore not used in semiconductor manufacturing lines to fabricate general semiconductor integrated circuits.
In IR arrays of a large scale integration of two dimensions such as solid state camera devices (or solid state imaging devices) in which detection elements for detecting IR radiation from objects are integrated, because integrated circuits of silicon are fabricated around peripheral sections of the integrated detection elements in order to read electrical signals, it can be carried out to fabricate the detection elements in IR detectors at a low cost and in high productivity when fabrication processes to fabricate the IR detection elements are introduced into fabrication processes of silicon semiconductor integrated circuits. Accordingly, it has advantages to use a semiconductor such as a polycrystalline silicon or an amorphous silicon as the resistance material 33 in the IR detector.
The mechanism of electric conduction in a polycrystalline silicon including dopants is designated by the sum of conductivity in the crystal and the conductivity through trap levels of crystal particles, as shown in one dimensional model of crystals connected in series disclosed by Seto et al. (see the Journal Of Applied Physics, vol.46, No.12, 1975, pp. 5247-5254, "The electrical properties of polycrystalline silicon films" by Seto, et al.).
When the number of carriers in the crystal is small, the carriers cannot fill all of the trap levels and form an electric barrier in the crystal particles. On the other hand, when the number of carriers in the crystal is greater, the carriers fill all of the trap levels and remaining carriers change the Fermi level in a crystal and decrease the level of the electric barrier. Where this electric barrier is designated by a reference character Ea (eV), the value of TCR can be designated by TCR=Ea/kT.sup.2.
When a polycrystalline silicon is used as the resistance material in a bolometer type IR detector, because the sensitivity of the IR detector is directly proportional to the value of TCR of the polycrystalline silicon, the level Ea of the electric barrier must be increased in order to form the IR detector with a high sensitivity. However, when the level Ea of the electric barrier becomes high, the electric conduction through the crystal is carried out based on the trap levels of the crystal particles. In this case, the magnitude of 1/f noise in the IR detector is greater. In general, because the performance of a bolometer type IR detector is measured based on the rate of a noise level in the level of output signal transferred from the IR detector, the performance of the bolometer type IR detector having the value of a greater 1/f noise becomes low. This problem is also caused when an amorphous silicon is used as the resistance material in the IR detector.
In addition, although the TCR value can be increased to approximately -8 [%/K] by decreasing the amount of dopants in the polycrystalline silicon as the resistance material, the resistance value is increased. In this case, it becomes difficult to match impedances between the bolometer type IR detector side and a signal processing circuit side for reading or receiving output signals transmitted from the bolometer type IR detector. This is also one of the problems involved in the conventional bolometer type IR detector described above.